The present invention relates to radio direction finding (DF) systems and, more particularly, to a DF-on-the-fly type compressive radio receiver capable of simultaneously and instantaneously determining the frequency and azimuthal and elevational position of multiple radio signal sources.
Direction finding (DF) systems are a well known and popular way to find the relative location of a radio transmitter. Such DF systems are commonly used by vessels at sea, for example, to provide an economic and reliable estimate of position via dead reckoning. That is, by drawing radii at the indicated azimuthal angle from two or more radio sources with known locations, the vessel's current position will be determinable as the point where the radii intersect.
In a situation as shown in FIG. 1, such DF systems can also be used to determine the presence of aircraft or vessels beyond the range of conventional radar. For example, DF systems deployed on a platform 110l, are capable of determining the azimuthal and angle .phi. between antenna 112 and any radio signal source 114 in the same horizontal plane. Likwise, the elevation angle .theta. between the receive antenna 112 and other radio transmitting platforms 116 and 118 which may be located in a horizontal plane above or below the plane of the receive antenna can also be determined.
In complex military situations, the large number of such remote radio transmitters as indicated at 114, 116 and 118 require nearly instantaneous identification and sorting to maximize the time available to formulate a proper strategic response. These systems are thus commonly described as having a "DF-on-the fly" capability.
U.S. Pat. No. 4,245,333 to Jelks is one such system employing a two-dimensional planar array of receiving elements positioned in rows and columns. Each row of elements is fed to a surface acoustic wave (SAW) device capable of producing a plurality of phase-shifted outputs. In turn, the phase-shifted outputs are fed to a scanning video device capable of indicating the direction and amplitude distribution of a radiating source.
In a so-called Adcock DF-on-the-fly system, a square array of four elements is employed. In such systems, well known in the art, the azimuthal angle can be instantaneously and accurately determined, overcoming the need for the many matching SAW networks inherent in the Jelks approach. Such Adcock antenna arrays are commonly limited to a three-octave bandwidth, however.
Returning again to the scenario of FIG. 1, as radio signal environments grow more dense, it becomes necessary to uniquely identify radio transmitters such as 116 and 120 which lie in approximately the same azimuthal plane, but in different elevational planes, as indicated by the angle .theta.. Prior radio signal sorting systems have not satisfactorily provided an estimate of elevational angle in an instantaneous and accurate manner. Typically, if such systems provided elevational position information at all, it was by first sorting signals by radio frequency and then employing a plurality of separate radio receivers and beamforming networks in a non-instaneous fashion.